It has been known for many years that elastic waves in a solid propagate according to several modes. One mode of propagation of elastic waves is a compressional wave, or "P-wave", in which particle motion within the solid is in a direction perpendicular to the wavefront. In another mode, the shear or "S-wave" mode, the particle motion within the solid is parallel to the wavefront. Compressional and shear waves travel at different velocities in a given solid and the ratio of these velocities, V.sub.p /V.sub.s, is a function of and hence indicative of the material in which the waves are propagating.
More recently, it has been found that in the field of seismic exploration in the search for oil, gas and other valuable minerals that the ratio of the velocities of the P-waves and the S-waves in a subterranean layer of rock is indicative of the composition of that rock and that if sufficiently accurate measurements of the ratio V.sub.p /V.sub.s are available to the geophysicist, he is in some cases able to predict the nature of the rock in a given subterranean layer. Since some rocks are more likely to contain these valuable minerals than others, the V.sub.p /V.sub.s ratio is therefore a tool in the search for oil and gas. See, Gregory, A. R., "Fluid Saturation Effects on Dynamic Elastic Properties of Sedimentary Rocks", Geophysics, Volume 41, Number 5, pages 895-921, October, 1976; Pickett, G. R., "Acoustic Character Logs and Their Application to Formation Evaluation", Journal of Petroleum Technology, June, 1963.
For many years, seismic exploration has been performed by generating an acoustic wave at or near the surface of the earth and detecting its reflection and retransmission to the earth's surface by one or more geophones also located at or near the earth's surface. Measurement of the travel time between emission and detection is indicative of the depth of the interface between layers of rock from which the wave is reflected; if enough such measurements are made, they can be used to generate a display indicative of subterranean structure, lithology and fluid content. Several techniques are used to generate the acoustic energy. A method which has proven increasingly useful in recent years is the "Vibroseis" technique in which a mechanical vibration is imparted to the earth typically by a truck having a plate lowerable from its chassis and adapted to be moved back and forth with respect to the earth at predetermined frequencies, so as to impart a mechanical energy to the earth. Most of the trucks used in Vibroseis prospecting have been designed to impart a vertical vibration to the earth so that the output recorded by geophones at the surface of the earth consists of compressional waves. However, apparatus has more recently been developed which imparts a side-to-side vibration to the earth, so that the geophone outputs correspond to shear waves. If both P-waves and S-waves are generated and reflected from common depth points in the earth's sub-surface, it is therefore possible to obtain a measurement of the ratio of their velocities, V.sub.p /V.sub.s, and from this to deduce the probable composition of the rock layer through which the waves pass. However, the difference in V.sub.p /V.sub.s with respect to different types of rocks is not very great, and accordingly very accurate measurements of V.sub.p /V.sub.s are necessary before reliable determinations of the sub-surface geology can be made. In particular, it is found that V.sub.p /V.sub.s, which can vary between about 1.20 and about 2.50 must be measured with an accuracy above about 0.1 or better, if the results are to be useful. No prior art technique is known which yields results consistently of this accuracy. Therefore, a need exists in the art for an improved method of determining V.sub.p /V.sub.s, as a tool for geophysical exploration.